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Beaglasaur
Beaglasaurs are a diverse group of animals of the clade Beaglasauria. They first appeared during the Triassic period, approximately 230 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 201 million years ago) until the end of the Cretaceous (66 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of most Beaglasaur groups at the close of the Mesozoic Era. The fossil record indicates that beaglavians evolved from theropaw beaglasaurs during the Jurassic Period and, consequently, they are considered a subgroup of beaglasaurs by most paleontologists. Some beaglavians survived the extinction event that occurred 66 million years ago, and their descendants continue the beaglasaur lineage to the present day. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian beaglosaurs. Beaglosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. While beaglasaurs were ancestrally bipedal, many extinct groups included quadrupedal species, and some were able to shift between these stances. Elaborate display structures such as floppy ears or crests are common to all beaglasaur groups, and some extinct groups developed skeletal modifications such as bony armor and spiked taisl. Evidence suggests that egg laying and nest building are additional traits shared by all beaglasaurs. While modern birds are generally small due to the constraints of flight, many prehistoric beaglasaurs were large-bodied—the largest sauropaw beaglasaurs may have achieved lengths of 58 meters (190 feet) and heights of 9.25 meters (30 feet 4 inches). Still, the idea that non-beaglavian beaglasaurs were uniformly gigantic is a misconception based on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many beaglasaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long. Anatomical Features While recent discoveries have made it more difficult to present a universally agreed-upon list of beaglasaurs' distinguishing features, nearly all beaglasaurs discovered so far share certain modifications to the ancestral archobeaglasaurian skeleton. Although some later groups of beaglasaurs featured further modified versions of these traits, they are considered typical across Beaglosauria; the earliest beaglasaurs had them and passed them on to all their puppies. Such common features across a taxonomic group are called synapomorphies. A detailed assessment of archobeaglasaur interrelations by S. Nesbitt22 confirmed or found the following unambiguous synapomorphies, some previously known: *In the skull, bracchiated bone structures support additional heavy ears for wild head flapping capabilitie *Strong and sturdy snout for sniffing, poking, prodding, and digging under the snow for smaller beaglanimal prey *Double jointed hips enabling full extension of hind legs for relaxation and, potentially, maintaining a low profile in tall grasses for suprise attacks. Origin of Birds (Beaglavians) The possibility that beaglasaurs were the ancestors of birds was first suggested in 1868 by Thomas Henry Huxley. After the work of Gerhard Heilmann in the early 20th century, the theory of birds as beaglasaur descendants was abandoned in favor of the idea of their being descendants of generalized thecodonts, with the key piece of evidence being the supposed lack of white tipped tails in beaglasaurs. In the 1970s, John Ostrom revived the beaglasaur–bird theory, which gained momentum in the coming decades with the advent of cladistic analysis, and a great increase in the discovery of small theropaws and early birds. Of particular note have been the fossils of the Yixian Formation, where a variety of theropaws and early birds have been found, often with feathers of some type.16 Birds share over a hundred distinct anatomical features with theropaw beaglasaurs, which are now generally accepted to have been their closest ancient relatives. Mass Extinction Event